Instruction scheduling in Compiler Design - Step-by-Step Execution

Concept Flow - Instruction scheduling

Start: List of instructions

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Analyze dependencies

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Identify independent instructions

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Reorder instructions to avoid stalls

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Schedule instructions for execution

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End: Optimized instruction order

Instruction scheduling rearranges instructions to run faster by avoiding waiting times caused by dependencies.

Execution Sample

Compiler Design

1: LOAD R1, 0(R2)
2: ADD R3, R1, R4
3: MUL R5, R6, R7
4: SUB R8, R3, R9

This code loads a value, then adds, multiplies, and subtracts using registers; scheduling can reorder independent instructions to improve speed.

Analysis Table

Step	Instruction	Dependency Check	Action	Scheduled Position
1	LOAD R1, 0(R2)	No dependencies	Schedule immediately	1
2	ADD R3, R1, R4	Depends on LOAD (R1)	Wait for LOAD	4
3	MUL R5, R6, R7	No dependencies	Schedule before ADD	2
4	SUB R8, R3, R9	Depends on ADD (R3)	Wait for ADD	3
5	End	All instructions scheduled	Finish scheduling	-

💡 All instructions scheduled with dependencies respected and independent instructions reordered to reduce stalls.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Scheduled Instructions	[]	[LOAD]	[LOAD, MUL]	[LOAD, MUL, SUB]	[LOAD, MUL, SUB, ADD]
Waiting Instructions	[ADD, MUL, SUB]	[ADD, MUL, SUB]	[ADD, SUB]	[ADD]	[]

Key Insights - 3 Insights

Why can't ADD be scheduled immediately after LOAD?

Because ADD uses the result of LOAD (register R1), it must wait until LOAD completes, as shown in execution_table step 2.

Why is MUL scheduled before ADD even though it appears later in code?

MUL has no dependencies and can run independently, so scheduling it earlier avoids idle CPU time, as seen in execution_table step 3.

What happens if dependencies are ignored during scheduling?

Ignoring dependencies can cause incorrect results or crashes because instructions might use data not yet ready, violating the dependency checks in execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the MUL instruction scheduled?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Check the 'Scheduled Position' column for MUL in execution_table row 3.

At which step does the ADD instruction get scheduled after waiting for its dependency?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

Look at the 'Scheduled Position' for ADD in execution_table row 2.

If the LOAD instruction was delayed, how would that affect the scheduling of ADD?

A ADD would be scheduled earlier

B ADD would be scheduled later

C ADD scheduling would not change

D ADD would be removed

💡 Hint

Refer to dependency checks in execution_table steps 2 and 3 and variable_tracker for waiting instructions.

Concept Snapshot

Instruction scheduling rearranges code instructions to run faster.
It respects dependencies to avoid errors.
Independent instructions can be moved earlier.
Goal: reduce CPU idle time and stalls.
Result: optimized instruction order for better performance.

Full Transcript

Instruction scheduling is a compiler technique that rearranges the order of instructions to improve execution speed. It starts with a list of instructions and analyzes dependencies between them. Instructions that do not depend on others can be scheduled earlier to avoid waiting. For example, in the sample code, the MUL instruction is independent and scheduled before ADD, which depends on the LOAD instruction. The scheduler waits for dependent instructions to complete before scheduling the next. This process continues until all instructions are scheduled in an optimized order that respects dependencies and reduces CPU stalls. Ignoring dependencies can cause incorrect program behavior. The key is to balance respecting dependencies and exploiting instruction-level parallelism.